1. Field of the Invention
The invention relates to an apparatus for reading an optically readable disc-shaped record carrier in which information comprising digital data signals and digital subcode signals is stored, the signal read from the record carrier being applied to a demodulator circuit for demodulating the information, extracting the digital data signals and the digital subcode signals, and generating a first clock signal which is locked to the channel-bit frequency of the signal being read, after which the digital data signal is applied to an error-correction circuit which comprises a buffer register into which the digital data signal is loaded with a frequency which is locked to said first clock signal and from which it is read with a frequency which is locked to a second clock signal supplied by an oscillator, the buffer register being capable of generating a rate error signal which is a measure of the instantaneous degree of filling of the buffer register.
2. Description of the Prior Art
Such apparatus is the well-known Compact-Disc-Digital-Audio player, which is marketed inter alia by N. V. Philips' Gloeilampenfabrieken under the type no. CD 100 and which is described in the magazine "Philips Technical Review", 40, No. 6, which entire issue is incorporated herewith by reference.
In this known apparatus the data signal comprises digital samples of an analog audio signal, which is recovered in the apparatus by means of a digital-to analog converter. The subcode signal contains address information and may also contain information about the content of the data signals. The buffer register is employed for correcting time-base errors in the data signal caused by, for example, eccentricities in the record carrier, because otherwise these time-base errors may become audible in the recovered analogue audio signal, whicch is annoying. In order to correct these time-base errors the frequency of the second clock signal is maintained highly constant, while the first clock signal follows the time base of the data signal being read. The data signal as recovered by the demodulation circuit comprises the digital audio signals together with inter alia the parity bits for error correction. Moreover, these data signals are interleaved, so that the errors are spread and are easier to correct. This correction and the restoration of the correct sequence of the audio signals is effected in the error-correction circuit which requires a memory circuit for this purpose. In the known apparatus the buffer memory is formed by a part of this memory circuit.
A potential use of the compact-disc digital audio system is the use of the record carrier as a read-only memory (ROM), in which case the signal on which the digital data signal is based need not be an analog audio signal, but may be a digital signal which is applied to a digital processing circuit instead of to an audio chain as in the known player.
For such a use it is found to be a disadvantage that the known apparatus generates a data signal with a corrected time base and a subcode signal with a non-corrected time base.
It is the object of the invention to provide an apparatus of the type specified in the opening paragraph, in which this problem is solved without modifying the demodulation circuit or the error-correction circuit, so that for the use the same integrated circuits may be employed for the demodulation circuit and the error-correction circuit as in an apparatus for the reproduction of audio signals.
To this end the invention is characterized in that the oscillator frequency is controlled by the rate error signal in such a sense that variations of the degree of filling rate of the buffer register are counteracted by variations of the read frequency which is dictated by the second clock signal.
The invention is based on inter alia the recognition of the fact that by this step the buffer memory is rendered inoperative with respect to the time-base correction of the data signal, so that the data signal remains in synchronism with the subcode signal, whereas the fact that the two signals no longer have a constant time base does not interfere with the further processing of the signals, because digital processing circuits generally accept substantially larger time base errors than those permissible in audio-signal reproduction. The step in accordance with the invention meets this objective because, oscillator replaces a reference crystal oscillator for generating the second clock signal in the known apparatus, while in the known apparatus the rate error signal is also externally available in order to control the speed with which the record carrier is driven. Therefore, the step in accordance with the invention does not necessitate any modification of the demodulation circuit or the error-correction circuit.
The apparatus in accordance with the invention may further be characterized in that the rate error signal is applied to the voltage-controlled oscillator via a low-pass filter having a bandwidth which is substantially larger than the frequency of variations in the first clock signal caused by eccentricities in the record carrier.
As a result of the step in accordance with the invention the rate error signal is no longer available as error signal for controlling the record-carrier drive.
With respect to the control of the record-carrier drive the apparatus in accordance with the invention may further be characterized in that the apparatus comprises a reference oscillator for supplying a reference clock signal derived from a demodulator and a comparator circuit for comparing the first clock signal with the reference clock signal for controlling the record-carrier drive.
An alternative to this apparatus may further be characterized in that the apparatus comprises a reference oscillator for supplying a reference clock signal and a comparator circuit for comparing the second clock signal with the reference clock signal for controlling the record-carrier drive.